The present invention relates to tape storage systems, and more particularly relates to an addition of error correcting capability to longitudinal position (LPOS) words encoded in the servo band of storage tapes, and a storage tape system and method of use that utilize the LPOS words with error correcting capability.
Tape storage systems for storing digital data, e.g., backup, are known. Tape storage systems provide an efficient, high capacity, cost-effective and reliable means for data backup. Linear Tape-Open (LTO) technology has developed through an effort to maximize capacity and performance of tape storage systems. LTO tapes use a tape format that includes longitudinally pre-written servo bands. The servo bands provide a timing-based band-following tape position error scheme. That is, the servo frames are encoded as LPOS words to provide the longitudinal position down the length of the tape. During operation, the servo band identification (ID) is identified by measuring the “phase” lead or lag between adjacent servo bands using start positions of servo frames.
In more detail, each servo band contains a repeated pattern of recorded flux transitions that occur in grouped bursts of 5, 5, 4 and 4 transition pairs. Each transition pair is referred to as a stripe and a grouping of 18 stripes is referred to as a frame. The timing (i.e., phase) between the sets of 5 bursts and 4 bursts provide the position information for band following. Select transition pairs within each of the 5 burst sets are phase-shifted to encode longitudinal position information (LPOS) into the server band. By detecting then decoding the phase encoded LPOS information, a tape storage system is able to determine the tape position relative to the landmarks lengthwise down the tape. The LPOS data are used as primary positional information for the tape servo control system to determine the starting and stopping of a tape, and to back-hitch the tape in order to position the read-write heads at the beginning of a data record at the required velocity and band position that allows the start of a new data transfer operation.
LPOS data typically cannot tolerate error. If a tape drive is reduced to a single channel because other servo heads have been smeared, or shorted, a single bit error on that channel can cause a stop write condition. For that matter, conventional LPOS words used on LTO tape storage schemes are not provided with error-correcting capabilities. As such, a single bit error in any one of the Sy, or L0 . . . L5 symbols could result in a stop write condition. In conventional tape storage operation, the likelihood that a single bit error can occur increases significantly if one servo head is not working, further increasing the possibility of stop write conditions.
Solutions have been attempted to reduce errors in LPOS words such as appending Reed-Solomon parity symbols to an LPOS word. Appending Reed-Solomon parity symbols, however, increases LPOS word length undesirably. Adding a base-14 checksum in an attempt to minimize error is also known, which adds a capability to identify, but not to correct errors. Another known solution is found in commonly-owned US Patent Application Publication No. US 2007/0044007, filed Aug. 17, 2005 (the '007 application), which teaches an LTO-based tape storage system and method, which provide error correction capability by initially encoding LPOS data via a set of even LPOS words, and a set of odd LPOS words. The encoded data comprising the sets of odd and even LPOS words are decoded by generating a set of syndrome bits for each of the LPOS words, and a determination is made as to whether there is error present in any one of the LPOS words based on its corresponding syndrome bits.
The most significant bit within each LPOS word symbol is encoded into a server sub-frame, such that an LPOS word containing 36 bits have a length of 36 servo frames. The LPOS words comprise Sy, L0, L1, L2, L3, L4, L5 and TX. In accordance with the '007 application, even LPOS words comprise Sy, L0, L1, L2, L3, X, Y, Tx, where L0 is 0, 2, . . . , 12, and odd LPOS words comprise Sy, L0, L1, L4, L5, X, Y and Tx, where LO is 1, 3, . . . , 13. There is no error in a particular LPOS word if a syndrome bit sum of the one LPOS word equals 0 or 1, and a single bit error if the sum equals 3. There is more than a single bit error if the syndrome bit sum of the LPOS word is 2-6.
The error correcting ability resides in the use of L4, L5 for even words and the use of L2, L3, for odd words used for X,Y, which are 8 bits of (d=0, k=3) run length-limited (RLL) constrained error correction code encoded as a, b, c, 1, d, e, f, 1, where a, b, c, d, e and f are six parity bits associated with a shortened extended Hamming code. The solution presented, however, is not ideal. To implement same requires a considerable modification of existing server encode-decode electronic circuits, and does not address effective error correction for ambiguous bits. Moreover, the known solution fails to safeguard or protect the sync mark symbol, Sy, the first symbols comprising each LPOS word, utilized to determine the beginning of an LPOS word. Nor is the known system able to add servo band ID information to each LPOS word.